1. Field of the Invention
The present invention relates to a periodic-pattern suppression method and unit, and more particularly to a method and unit for reducing and removing a stripe pattern (including a moire component), corresponding to a stationary grid, from an image photographed, for example, by use of the stationary grid, the image having the stripe pattern superposed on the image of a subject.
2. Description of the Related Art
Radiation recording-reproducing systems have been proposed in Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395, 55(1980)-163472, 56(1981)-164645, 55(1980)-116340, etc. These systems utilize a storage-type phosphor (stimulatable phosphor), which stores part of radiation energy when irradiated with radiation, such as X-rays, etc., and emits photostimulated luminescent light according to the amount of the stored radiation energy when irradiated with excitation light such as visible light, etc. The radiation image of a subject, such as a human body, etc., is temporarily photographed and recorded on a storage-type fluorescent sheet. Then, excitation light, such as laser light, etc., is scanned on the storage-type fluorescent sheet to obtain photostimulated luminescent light. The obtained photostimulated luminescent light is photoelectrically read out and converted to an electrical image signal by a reading means such as a photomultiplier, etc. Based on the image signal, the radiation image of the subject is output as a visible image to a recording material such as a photosensitive material, etc., or to a cathode-ray tube (CRT) display unit, etc.
There are cases where, when photographing and recording the radiation image of a subject on the above-mentioned storage-type fluorescent sheet, photographing is performed with a stationary grid disposed between the subject and the fluorescent sheet. In the stationary grid, lead plates, which do not transmit radiation, and aluminum or wood plates, which easily transmit radiation, are alternately disposed at fine pitches of about 4 plate/mm so that radiation scattered by the subject is not irradiated to the fluorescent sheet. If photographing is performed using this stationary grid, radiation scattered by the subject will become less liable to be irradiated to the fluorescent sheet and therefore the contrast of the radiation image of the subject can be enhanced. However, the grid image in the form of a fine stripe pattern corresponding to the stationary grid, along with the subject image, is recorded.
Because of this, the applicant of this application has proposed, in Japanese Unexamined Patent Publication No. 10(1998)-164737, a method of obtaining a stripe-reduced image which is easy to observe, by applying a filtering process for removing a spatial frequency component corresponding to the stripe pattern of a stationary grid. In this method, for example, in the case where the grid array pitch (distance between the centers of adjacent grid rods) of a stationary grid (hereinafter referred to as a grid pitch) is 4 grid-line/mm, a stripe pattern develops in a spatial frequency band near 4.0 cycle/mm. In order to remove the stripe pattern, a filtering process is performed by a filter which removes or reduces its response at this frequency band.
However, the filter employed in the method in the aforementioned publication No. 10(1998)-164737 suppresses and removes not only the stripe pattern of the stationary grid but also the high frequency component contained in the original image, because its response at a frequency component near a spatial frequency corresponding to the grid pitch of a stationary grid used, and its response at a high frequency component greater than that, are made zero. Thus, there is a problem that an image will be reduced in sharpness.